1. Field of the Invention
This invention relates to heat shrinkable material which is suitable for use in covering terminals or joints of power cables, telegraph cables, etc., and joints of various types of fluid transport pipes.
2. Description of the Prior Art
Heat shrinkable materials conventionally used for covering terminals or joints of electric cables and transport pipes include: (1) a material comprising a heat shrinkable substrate and a layer of an adhesive provided on the whole surface of one side of the substrate, said adhesive being capable of adhering at ordinary temperatures; and (2) a material comprising a heat shrinkable substrate and a layer of an adhesive provided on the whole surface of one side of the substrate, said adhesive being incapable of adhering at ordinary temperature, but being capable of adhering when heated. The ordinary temperature used herein means the temperatures where the adhesive is used, and is generally from about -20.degree. to 50.degree. C.
The former heat shrinkable material has high impact strength, and exhibits good adhesion at low temperatures (e.g., about 10.degree. C. or less), but their adhesion strength is liable to decrease at higher temperatures.
The latter heat shrinkable material seldom gives rise to the problem of reduction in adhesion strength as is encountered with the former material. However, where it is used for covering, for example, steel pipes for transportation of fluid, adhesion breakage at the interface of the surface of the steel pipe and the adhesive occurs readily upon an impact from outside onto the area covered with the material. This leads to interfacial peeling of the adhesive, since the adhesive has relatively low elasticity. This tendency becomes more marked as the temperature decreases.
Pipes for the transportation of fluids are often protected from corrosion by passing a small quantity of an electric current therethrough. When the pipes are covered with known heat shrinkable materials, however, a small amount of hydrogen gas is produced by the electric current, thus causing peeling of the adhesive (i.e., cathodic peeling).
Furthermore, it has been found that once the interfacial peeling or cathodic peeling occurs, it is propagated and the peeled area extends rapidly.